So, the widely taught reason necessitating the 13th Amendment, that the states of the former Confederacy under reconstruction were not accepting former slaves as citizens, is not exactly the whole truth, is it now? Fact was, slavery still existed in the Union slave states, slavery wasn't abolished by the war, slaves weren't all "freed" by the defeat of the Confederacy. Slaves remained in bondage in Maryland, Delaware, Kentucky and Missouri for eight months longer.
Why is that, if the Union fought to free the slaves as so many mistakenly insist? The reason for this is that the Union did not fight to free the slaves, that was a military ploy devised during the war to foment rebellion in the Confederacy. After the war's end, this wartime proclamation created certain difficulties in Union states, that required amending the Constitution of the United States to resolve.
posing the war as one to end slavery is what kept the Brits and the French from active involvement on the side of the Confederacy. A very pragmatic and smart move by the Lincoln administration.